18 File handling

1. 18 File handling CE00858-1: Fundamental Programming Techniques 18 File handling

2. Objectives In this lecture, we will: • open files for input and output • close files after processing • read data from file • write results to file 18 File handling

3. Why use files? • the data in programs written so far has either been: • hard coded into the program • read from the keyboard using a Scanner object • more useful to: • read data from file • store results in a file int width = 4; Scanner kybd = new Scanner(System.in); width = kybd.nextInt(); 18 File handling

4. Using input files • must import java.io.* • create a Scanner object to read from a file • need the name of the file to read in place of System.in • file must be in same folder as Java program or checked exception will occur • checked exception must be handled • can have any number of Scanner objects • one to read from each file • one to read from keyboard • when processing done all files should be closed 18 File handling

5. Reading from file • Scanner object methods are used to read from file in same way as they read from keyboard: • nextInt() • nextDouble() • next() • nextLine() • hasNext() method: • returns true if there is more data to read • returns false when end of file is reached • can use hasNext() method in a while loop to process all data in a file 18 File handling

6. Using input files example - AverageAge • program to read in names and ages from a data file called in.txt • calculate and output the average age to the screen • assume that all data is valid 18 File handling

7. AverageAge analysis • what data is used? • sum: double calculated in program to total ages • count: integer calculated in program to count number of people • age: double input from file • name: string input from file • what operations are needed? • iteration to process contents of file • what operations are performed once before the loop? • create Scanner and link it to in.txt • initialise count and sum to 0 • how many times is the loop performed? • loop performed until end of file reached 18 File handling

8. AverageAge analysis continued • what operations are repeated inside the loop? • read name and age • add age to sum • add 1 to count • what operations are performed once after the loop • close in.txt • calculate and output average • what exceptions may occur • file not found • error reading from file • division by 0 if no data input 18 File handling

9. AverageAge.java //input ages from file and calculate average import java.util.*; import java.io.*; public class AverageAge { public static void main (String [] args) { double sum = 0; int count = 0; try { //create Scanner linked to input file Scanner in = new Scanner(new File("in.txt")); String name; double age; continued on next slide 18 File handling

10. //read until end of file reached while (in.hasNext()) { name = in.next(); age = in.nextDouble(); sum = sum + age; count++; } //close file in.close(); double average = sum / count; System.out.println("The average age is: " + average); } //handle any errors catch(ArithmeticException e) { System.out.println("Cannot calculate average - no data"); } catch(IOException e) { System.out.println("Error reading from file"); } } } 18 File handling

11. Using output files • must import java.io.* • create a PrintWriter object to write to a file • need the name of the file to write to • file will be in same folder as Java program • if file already exists contents will be overwritten • if file doesn't exist, new one is created • checked exception must be handled • can have any number of PrintWriter objects 18 File handling

12. Writing to file • PrintWriter object methods are used to write data to file: • print() • println() • printf() 18 File handling

13. Using output files example - OddsEvens • program to read in positive integers entered at the keyboard until -1 is entered • output all even numbers to a file called even.txt • output all odd numbers to a file called odd.txt • assume that all data is valid 18 File handling

14. OddsEvens analysis • what data is used? • num: integer input by user • what operations are performed? • iteration to process all numbers input • what operations are performed once before the loop? • create PrintWriter and link it to even.txt • create PrintWriter and link it to odd.txt • read num • how many times is the loop performed? • loop performed until -1 entered 18 File handling

15. OddsEvens analysis continued • what operations are repeated inside the loop? • output num to even.txt if even • output num to odd.txt if odd • read num • what operations are performed once after the loop • close even.txt and odd.txt • what exceptions may occur • error writing to file 18 File handling

16. OddsEvens.java //input numbers and output odd ones to odd.txt, even ones to even.txt import java.util.*; import java.io.*; public class OddsEvens { public static void main (String [] args) { try { //create PrintWriters linked to output files Scanner kybd = new Scanner(System.in); PrintWriter even = new PrintWriter("even.txt"); PrintWriter odd = new PrintWriter("odd.txt"); int num = kybd.nextInt(); //read until -1 entered while (num != -1) { continued on next slide 18 File handling

17. if (num % 2 != 0) { odd.println(num); } else { even.println(num); } num = kybd.nextInt(); } //close files odd.close(); even.close(); } catch(IOException e) { System.out.println("Error writing to file"); } } } 18 File handling

18. Handling exceptions • opening, reading, writing and closing files can generate a checked exception • if these are not handled, the compiler generates an error 18 File handling

19. Summary In this session we have: • opened and closed files • read data from a file • written results to a file 18 File handling